224 GOLD IN SCIENCE AND IN INDUSTRY. 



-pose at or helow. their transition, temperature. The significance of 

 this fact, I venture to tliink, entitles it to more tlian a passing refer- 

 ence. It appears to me to mean that the transition from amorphous 

 to crystalline is entitled to take its place with the other great changes 

 of state, solid to liquid, liquid to gas, for like these, it marks a 

 change in the molecular activity which occurs when a certain tem- 

 perature is reached. It is entitled to take this place because there 

 is every indication that the change is as general in its nature as the 

 other changes of state. Compare it, for instance, with the allo- 

 tropic changes with which chemists have been familiar. These are 

 for the most part changes which are special to particular elements 

 or compounds and are usually classed with the chemical properties 

 by which the substances may be distinguished from each other. 

 Very different is the amorphous crystalline change, for although 

 in particular cases it may have been observed and associated with 

 allotropic changes, jet the causes of its occurrence are more deeply 

 founded in the relations between the molecules and the heat energy 

 by which their manifold properties are successively unfolded as tem- 

 perature is raised from the absolute zero. At this transition point 

 we find ourselves face to face with the first stirrings of a specific 

 directive force by which the blind cohesion of the molecules is 

 ordered and directed to the building up of the most perfect geo- 

 metric forms. It is hardly possible any longer to regard the stability 

 of a crystal as static and inert and independent of temperature. 

 Rather must its structure and symmetry be taken as the outward 

 manifestation of a dynamic equilibrium between the primitive co- 

 hesion and the kinetic energy imparted by heat. Even before the 

 discovery of a definite temperature of transition from the amorphous 

 to the crystalline phase we had in our hands the proofs that in cer- 

 tain cases the crystalline state can be a state of dynamic rather than 

 of static equilibrium. The transition of sulphur from the rhombic 

 to the jjrismatic form supplies an example of crystalline stability 

 which persists only between certain narrow limits of temperature. 

 Within these limits the crystal is a " living crystal," if one may 

 borrow an analogy from the organic world. It can still grow^, and 

 it will under proper conditions repair any damage it may receive. 



The passage of the same substance through several crystalline 

 phases, each only stable over a limited range of temperature, strongly 

 supports the general conclusion drawn from the existence of a sta- 

 bility temperature between the amorphous and crystalline phases, 

 namely, that the crystalline arrangement of the molecules requires for 

 its active existence the particular kind or rate of vibration corre- 

 sponding with a certain range of temperature. Below this point the 

 crystal may become to all appearance a mere pseudomorph with no 

 powers of active growth or repair. But these powers are not ex- 

 tinct — thev are only in abeyance ready to be called forth under the 



